Microwave series switch biasing circuit

ABSTRACT

A series biasing circuit includes a three terminal filter circuit within an outer conductor defining a cavity. The filter circuit has two signal terminals extending through and insulated from the wall of the cavity and common terminals connected to the cavity. A conducting wire extends through and is insulated from the outer conductor and into the cavity and connects to the filter circuit at a point which is capacitively insulated from the outer conductor. Portions of the inner conductors coact with the outer conductor within the cavity to define substantially inductive transmission line segments. These substantially inductive segments coact with substantially capacitive elements to define a substantially reflectionless filter circuit.

United States Patent (72] Inventor Peter A.Rlu1

Dedhlm, Mass. [21 Appl. No 826.510 [22] Filed May 21.1969 [45] PatentedJuly 13,1971 [73] Assignee Alpha Industries Inc.

Newton Upper Falls, Mass.

[54] MICROWAVE SERIES SWITCH BIASING CIRCUIT 9 Claims, 4 Drawing Figs.

[52] US. Cl 333/73 R, 333/97 5 [Si] lnt.Cl H0lp H20, HOlp 1/10 [50]FieldotSearch .4 v 7 333/173, 73 C 73 S. 97, 97 S [56] References CitedUNITED STATES PATENTS 3,230,386 1/1966 Riebman et al H 333/97 X3,475,700 10/1969 Ertel 333/7 3,503,015 3/1970 Coraccio et al a t t333/7 OTHER REFERENCES R V Garver. Theory of TEM Diode Switching," IRETRANS. ON MTT. May. 1961. pg. 232- 234 relied on 333- 978 PrimaryExaminer-Herman Karl Saalbach Assistant Examiner- Paul L, GenslerAttorney-Charles Hieken ABSTRACT: A series biasing circuit includes athree terminal filter circuit within an outer conductor defining acavity. The filter circuit has two signal terminals extending throughand insulated from the wall of the cavity and common terminals connectedto the cavity, A conducting wire extends through and is insulated fromthe outer conductor and into the cavity and connects to the filtercircuit at a point which is capacitively insulated from the outerconductor. Portions of the inner conductors coact with the outerconductor within the cavity to define substantially inductivetransmission line segments. These substantially inductive segments coactwith substantially capacitive elements to define a substantiallyreflection less filter circuit.

PATENTEU JULISISH 3,593,222

d brig? LW INVENTOR.

CB 20 PETER A. RIZZI MICROWAVE SERIES SWITCH BIASING CIRCUIT BACKGROUNDOF THE INVENTION The present invention relates in general to seriesbiasing circuits and more particularly concerns a novel broadband seriesbiasing circuit of high electrical performance and small physical formwhich is relatively easy and inexpensive to fabricate in large and smallquantities with uniformly high quality.

Most methods of biasing in microwave circuits locate the biasing circuitin the microwave package itself. While this technique has not foreclosedsatisfactory biasing, it does introduce fabrication, electrical andmechanical problems.

Also switching elements, such as crystals, normally require a return forcrystal current to the conductor casing, usually maintained at ground orreference potential. The mechanical structure for such networks is notonly complex, but also presents considerable problems in RF impedancematching.

Where the biasing circuitry or the return for the switchable elementscannot be located within the microwave package because of mechanical orelectrical constraints, a series biasing circuit is needed. Prior artseries biasing circuits have a number of disadvantages. The mechanicalstructures have been relatively large and complex, presentingconsiderable problems in packaging and in RF impedance matching. Also,the bandwidth of these packages has been inherently small, therebylimiting the bandwidth of the operating microwave system as a whole.

Accordingly, it is an important object of this invention to provide aseries biasing circuit which is substantially reflectionless over arelatively broad band of frequencies.

It is another object of the invention to provide a series biasingcircuit in accordance with the preceding object which provides a returnfor crystal current.

It is another object of the invention to provide a series biasingcircuit in accordance with the preceding object which may still functioneffectively as a filter for other purposes.

It is another object of the invention to provide a series biasingcircuit capable of handling relatively large quantities of RF and biaspower while introducing relatively little reflection in microwavefrequencies of interest.

It is another object of the invention to provide a series biasingcircuit susceptible to sealed or unsealed operation which is relativelyeasy and inexpensive to fabricate in large and small quantities withuniformly high quality.

SUMMARY OF THE INVENTION According to the invention, a substantiallyreflectionless filter circuit, comprising substantially reactive seriesand shunt elements, having three terminals is placed within an outerconductor defining a cavity, so that its series signal terminals extendbeyond the outer conductor defining the cavity and are insulatedtherefrom. The remaining terminal of the filter circuit is connectedwithin the cavity to the outer conductor. A conducting wire extendsthrough and is insulated from the outer conductor and into the cavityand connects to the sub stantially reflectionless filter circuit at apoint which is capacitively insulated from the outer conductor. Withinthe cavity, portions of the inner conductors coact with the outerconductor forming substantially inductive transmission line segments.Substantially capacitive elements included within the cavity coact withthe substantially inductive segments to comprise an essentially reactivebiasing network. A first substantially capacitive element within thecavity serially connects the signal terminals of the circuit. The firstsubstantially capacitive element is interposed between substantiallyinductive transmission line segments, thus creating a high pass filterconfiguration. The substantially inductive transmission line segmentsconnect with the outer conductor by second substantially capacitiveelements. The second substantially capacitive elements are chosen sothat substantially all of the RF signal is passed through to groundwhile the bias signal is effectively blocked.

Numerous other features, objects and advantages of the invention willbecome apparent from the following specification when read in connectionwith the accompanying drawings in which:

BRIEF DESCRIPTION OF THE DRAWINGS FIG. I is a longitudinal section viewof a preferred embodiment of the invention;

FIG. 2 is a sectional view through section line 2-2 of FIG. 1',

FIG. 3 is a schematic circuit diagram of a preferred embodiment of theinvention in series with an element to be biased; and

FIG. 4 is the dual of the circuit of FIG. 3 and represents anotherembodiment of the invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS With reference now tothe drawings and more particularly to FIG. I thereof, there is shown alongitudinal sectional view of a preferred embodiment of the inventionin which the series terminals I0 and 12 of the substantiallyreflectionless filter circuit are diagrammatically represented. Outerconductor casing 20 defines a cavity for the substantiallyreflectionless filter circuit. Conductors II and 13 respectively connectto signal terminals 10 and 12 and extend through, and are insulated fromcasing 20, and into the cavity connecting to opposite terminals ofseries capacitor 30. Capacitor 30 is insulated from outer conductorcasing 20 by insulating means 31. Within the cavity, conductors I5 and17 respectively connect conductors II and I3 to terminals of capacitors32 and 34 respectively. Capacitors 32 and 34 have their remainingterminals connected to outer conductor casing 20. Conductor 19 extendsthrough and is insulated from outer conductor casing 20 and into thecavity connecting with capacitor 34 at a terminal capacitively insulatedfrom outer conductor casing 20. Conductors I5 and I7 coact with outerconductor 20 to define substantially inductive transmission linesegments.

FIG. 2 is a sectional view through the embodiment of FIG. I betterillustrating the relationship among capacitor 30, insulator 31 andconductors I I and I3 and casing 20.

With reference now to FIG. 3, there is shown a schematic circuit diagramof the embodiment of FIGS. I and 2 showing the series and shunt legs,biased elements, as for example, a diode network 45, in series and thebias source 50. Capacitor 30 is connected in series with conductors IIand I3 and terminals I0 and I2. The shunt legs comprise a seriescombination of inductive and capacitive elements, conductor 15 acting asan inductive element and capacitor 32 as a capacitive element in one legand conductor I7 acting as an inductive element and capacitor 34 as acapacitive element in the other leg. The shunt legs are connected toconductors I I and I3, respectively, and are both attached to casing 20at the other end to define a common terminal. Conductor 18 also isconnected to casing 20. Conductor 19 connects to the shunt leg at apoint A capacitively insulated from the common terminal and supplies thebias input to the circuit to be switched. Bias is supplied by biassource 50 across conductors Ill and 19. The combination of diodes 40 and41 is connected in series with the circuit and form the biased network45.

Referring to FIG. 4 there is shown the dual of the filter circuit ofFIG. 3. It is known that if it is desired to obtain the driving pointadmittance having the same characteristic on an admittance basis as theimpedance of the known circuit, it is only necessary to make thefollowing changes:

I. Change each capacitor having a value C in farads to an inductorhaving the same inductance L numerically in henries;

2. Change each inductance having a value L in henries to a capacitorhaving the same numerical value C in farads;

3. Change series-connected elements to shunt-connected elements; and

4. Change shunt-connected elements to series-connected elements.

This is accomplished in a circuit of FIG. 4. Thus, the filter circuitconsists of serially connected parallel combinations of C and L-,' withconductors 11 and 13 and terminals 10 and 12. Inductive element Lcomprises the shunt leg which is connected at one end between theparallel combination of the series reactive elements to terminal 40 andhas its other end connected to the common terminal 20. Again bias issupplied at a point A capacitively insulated from outer conductor 20 byconductor 19. This may be accomplished by inserting an RF bypasscapacitor C in series with inductive element L,.

Having described the embodiments and the physical arrangement of thecircuit components, it is appropriate to consider the techniques anddesign criteria used. The schematic circuit diagram FIG. 3 illustrates aseries m-derived high pass filter in IT-SCCllOII form in series with adiode network and a bias source. The filter is designed to present asubstantially matched impedance to the biased RF circuit over thefrequency range of interest. The schematic circuit diagram of FIG. 4shows another shunt m-derived high pass filter of T-section form. Thecomponents of this filter circuit are also chosen to present asubstantially matched impedance to the biased RF circuit.

The capacitors in the shunt legs of the embodiment of FIG. 3 of theinvention may be chosen so that substantially all of the RF signal ispassed to the outer conductor casing while effectively blocking the biassignal at that point. Thus the substantially inductive transmission linesegments may be maintained substantially at RF ground potentialrendering the biasing circuit virtually independent of the bias signalnetwork. Consequently, the RF signal passing through the circuit will bevirtually independent of the configuration and variations of the biasinsertion network.

The inductance value of the substantially inductive transmission linesegments may be calculated from the well-known equation:

X =Z tanBi where the capacitors in the shunt legs are effective shortcircuits to RF frequencies and B=21rlk and L is the length of theconductor. It is preferred that, in the range of operation, the lengthof the inductive segment be substantially less than the wavelength,preferably approximately a quarter wavelength at the highest frequencyofinterest. Then:

X =wLZZ (wl/v) where u is the velocity of propogation of theelectromagnetic wave. Thus,

LZZol/V The appropriate values of the respective capacitances andinductances may then be derived by using filter circuit formulas.

In a specific embodiment of the invention, 50 ohm type coaxial terminalswere used with an outer conductor casing of Kovar material 0100 inchlong. The cavity formed within the outer conductor was 0.100 by 0.250inch. 0.001 by 0.005 inch wire was used for the series inner conductorsof the series mderived rr-section high pass filter within the cavity. Al pf capacitor was used in the series leg of the filter circuit. 0.001inch diameter wire was used to create substantially inductive elementsof the shunt legs and 50 pf capacitors were also used in the shunt legsto create substantially capacitively reactive elements. 0.001 inchdiameter wire was used within the cavity for supplying the bias signalto the circuit and extended through and was insulated from the casing toan axial terminal outside the cavity.

The series biasing circuit operated over a frequency range of L to 12GHz. with a maximum of SWR of 2.0. The device has an insertion loss of0.5 db. over that range and the maximum power rating of watts.

An important feature of the invention is the adaptability of thestructure to accommodating different types of coaxial terminals atdifferent locations. FIG. 1 shows signal terminals being brought out inopposite points on longitudinal axis of the biasing circuit but theinvention operates equally well with the signal terminals and spacequadrature of at other suitable angles.

FIG. 3 shows the series biasing circuit in series with a diode network.The biasing circuit may be used to bias any type of diode, diode circuitor other network which requires a bias signal.

The invention is illustrated with rectangular cavity formed by the outerconductor casing. The cavity may be cylindrical or any other suitableshape. The series biasing circuit may also be constructed of twoparallel plates forming a transmission line with the filter circuitbeing mounted on one or both of the plates. The invention may also beconstructed in microstrip or any other TEM waveguide configuration.

Other modifications and uses of and departures from the specificembodiment as described herein may be practiced by those skilled in theart without departing from the inventive concepts. Consequently, theinvention is to be construed as embracing each and every novel featureand novel combination of features present in or possessed by theapparatus and techniques herein disclosed and limited solely by thespirit and scope of the appended claims.

What I claim is:

l. A broadband series biasing circuit operative over a predeterminedmicrowave frequency range comprising,

means defining first and second terminal pairs each having a signalterminal and a reference terminal,

first conducting means defining a cavity and normally maintained at areference potential intercoupling reference ones of each of saidterminal pairs,

means wherein said cavity defining a substantially reflectionless highpass filter circuit that is one of 1: and T networks formed ofsubstantially inductive and substantially capacitive elementsintercoupling said signal terminals and having a biasing substantiallycapacitive element having one terminal contacting said first conductingmeans and its other terminal in series with a first substantiallyinductive element that is a lead connected to at least one othercapacitive element connected to the junction of a second inductiveelement and said first signal terminal, said substantiallyreflectionless high pass filter circuit presenting a substantiallyconstant predetermined impedance to said first and second terminal pairsover said predetermined microwave frequency range, and means defining abiasing signal terminal extending through and insulatedly separated fromsaid first conducting means and connected to said other terminal, saidreflectionless high pass filter circuit comprising means forestablishing an RF path between the signal terminals of said first andsecond terminal pairs and a conductive path for carrying a DC biaspotential between said biasing signal terminal and at least one of saidsignal terminals,

said signal terminals extending through and insulatedly separated fromsaid first conducting means,

2. A broadband series biasing circuit in accordance with claim 1 whereinsaid substantially refiectionless high pass filter circuit is a 11network having said one other substantially capacitive elementintercoupling said signal terminals, a first series circuit comprisingsaid second substantially inductive element and a third substantiallycapacitive element intercoupling one of said signal terminals with saidfirst conducting means and a second series circuit comprising said firstsubstantially inductive element and said biasing substantiallycapacitive element intercoupling the other of said signal terminals withsaid first conducting means.

3. A broadband series biasing circuit in accordance with claim 1 whereinsaid substantially reflectionless filter circuit is a T network having afirst parallel circuit comprising said second substantially inductiveelement and said one other substantially capacitive element in serieswith a second parallel circuit comprising a third substantiallyinductive element and a third substantially capacitive elementintercoupling said signal terminals and a series circuit comprising saidfirst substantially inductive element and said biasing substantiallycapacitive element intercoupling the junction of said first and secondparallel circuits with said first conducting means.

4. A broadband biasing circuit in accordance with claim 2 whereinconductors insulatedly separated from said first conducting meanscomprise said substantially inductive elements and coact with said firstconducting means to define transmission line segments correspondingsubstantially to a quarter wavelength at a predetermined highestfrequency of interest in said predetermined microwave frequency rangehas been inserted.

5. A broadband biasing circuit in accordance with claim 3 whereinconductors insulatedly separated from said first conducting meanscomprise said substantially inductive elements and coact with said firstconducting means to define transmission line segments correspondingsubstantially to a quarter wavelength at a predetermined highestfrequency of interest in said predetermined microwave range has beeninserted.

6. A broadband biasing circuit in accordance with claim 4 and furthercomprising unilaterally conducting means to be biased intercoupling atleast one of said signal terminals DC coupled to said biasing signalterminal with said first conducting means,

7. A broadband biasing circuit in accordance with claim 5 and furthercomprising unilaterally conducting means to be biased intercoupling atleast one of said signal terminals DC coupled to said biasing signalterminal with said first conducting means.

8. A broadband biasing circuit in accordance with claim 6 and furthercomprising a source of biasing potential intercoupling said biasingsignal terminal with said first conducting means.

9. A broadband biasing circuit in accordance with claim 7 and furthercomprising a source of a biasing potential intercoupling said biasingsignal terminal with said first conducting means.

1. A broadband series biasing circuit operative over a predeterminedmicrowave frequency range comprising, means defining first and secondterminal pairs each having a signal terminal and a reference terminal,first conducting means defining a cavity and normally maintained at areference potential intercoupling reference ones of each of saidterminal pairs, means wherein said cavity defining a substantiallyreflectionless high pass filter circuit that is one of pi and T networksformed of substantially inductive and substantially capacitive elementsintercoupling said signal terminals and having a biasing substantiallycapacitive element having one terminal contacting said first conductingmeans and its other terminal in series with a first substantiallyinductive element that is a lead connected to at least one othercapacitive element connected to the junction of a second inductiveelement and said first signal terminal, said substantiallyreflectionless high pass filter circuit presenting a substantiallyconstant predetermined impedance to said first and second terminal pairsover said predetermined microwave frequency range, and means defining abiasing signal terminal extending through and insulatedly separated fromsaid first conducting means and connected to said other terminal, saidreflectionless high pass filter circuit comprising means forestablishing an RF path between the signal terminals of said first andsecond terminal pairs and a conductive path for carrying a DC biaspotential between said biasing signal terminal and at least one of saidsignal terminals, said signal terminals extending through andinsulatedly separated from said first conducting means,
 2. A broadbandseries biasing circuit in accordance with claim 1 wherein saidsubstantially reflectionless high pass filter circuit is a pi networkhaving said one other substantially capacitive element intercouplingsaid signal terminals, a first series circuit comprising said secondsubstantially inductive element and a third substantially capacitiveelement intercoupling one of said signal terminals with said firstconducting means and a second series circuit comprising said firstsubstantially inductive element and said biasing substantiallycapacitive element intercoupling the other of said signal terminals withsaid first conducting means.
 3. A broadband series biasing circuit inaccordance with claim 1 wherein said substantially reflectionless filtercircuit is a T network having a first parallel circuit comprising saidsecond substantially inductive element and said one other substantiallycapacitive element in series with a second parallel circuit comprising athird substantially inductive element and a third substantiallycapacitive element intercoupling said signal terminals and a seriescircuit comprising said first substantially inductive element and saidbiasing substantially capacitive element intercoupling the junction ofsaid first and second parallel circuits with said first conductingmeans.
 4. A broadband biasing circuit in accordance with claim 2 whereinconductors insulatedly separated from said first conducting meanscomprise said substantially inductive elements and coact with said firstconducting means to define transmission line segments correspondingsubstantially to a quarter wavelength at a predetermined highestfrequency of interest in said predetermined microwave frequency rangehas been inserted.
 5. A broadband biasing circuit in accordance withclaim 3 wherein conductors insulatedly separated from said firstconducting means comprise said substantially inductive elements andcoact with said first conducting means to define transmission linesegments corresponding substantially to a quarter wavelength at apredetermined highest frequency of interest in said predeterminedmicrowave range has been inserted.
 6. A broadband biasing circuit inaccordance with claim 4 and further comprising unilaterally conductingmeans to be biased intercoupling at least one of said signal terminalsDC coupled to said biasing signal terminal with said first conductingmeans,
 7. A broadband biasing circuit in accordance with claim 5 andfurther comprising unilaterally conducting means to be biasedintercoupling at least one of said signal terminals DC coupled to saidbiasing signal terminal with said first conducting means.
 8. A broadbandbiasing circuit in accordance with claim 6 and further comprising asource of biasing potential intercoupling said biasing signal terminalwith said first conducting means.
 9. A broadband biasing circuit inaccordance with claim 7 and further comprising a source of a biasingpotential intercoupling said biasing signal terminal with said firstconducting means.